1. Field of the Invention
The present invention relates to a device and method for limiting peaks of a signal and, more particularly, to a device and method for reducing the peak-to-average (P/A) ratio of an input signal without generating significant out-of-band emissions.
2. Description of Related Art
Power amplifiers have non-linear characteristics. The cost of power amplifiers is determined by the size of their linear range. The non-linear characteristic of conventional power amplifiers causes out-of-band spectral artifacts, e.g., spectrum distortions, splatters, spectrum spreading, etc. By reducing the peaks of a signal input to the power amplifier, the P/A ratio of the input signal is reduced, allowing the amplifier to output a large average power.
A hard clipping process is one of the conventional ways to reduce the P/A ratio of an input signal. The effect of the hard clipping process is that a noise-like signal is added to the input signal to generate a hard clipped signal. The algorithm of the hard clipped signal is as follows, wherein Vin represents the input signal, Vclip represents the clipping threshold signal, and Vout represents the hard clipped signal.
If Vinxe2x89xa7Vclip, then Vout=Vclip, or
if Vinxe2x89xa6xe2x88x92Vclip, then Vout=xe2x88x92Vclip,
else Vout=Vin.
A hard clipped signal has abrupt edges and sharp peaks. The abrupt nature of the hard clipping process and the short time duration of clipped edges generate significant out-of-band spectral artifacts, such as spectrum distortions, splatter, and spectrum spreading. For example, when an input signal, whose spectrum is illustrated in FIG. 1, is peak-limited by the conventional hard clipping process, the effect in the frequency domain is to add the spectrum of a noise-like signal, shown in FIG. 2A, to the input signal spectrum of FIG. 1. The spectrum of the final result of the hard clipping process is then as shown in FIG. 2B. Although the spectrum response of the input signal outside of about 0.3 frequency units is low, the spectrum of the hard clipped signal outside of about 0.3 frequency units is much higher than that of the input signal. Therefore, the hard clipping process is generally undesirable in reducing the P/A ratio of an input signal.
A window clipping process discussed in U.S. Pat. No. 5,287,387 is another conventional method of reducing the P/A ratio of a signal. In the window clipping process, an attenuating window signal is multiplied with the input signal to generate an attenuated signal. Portions of the attenuating window signal corresponding to the peak areas of the input signal have a value of less than one, and all other portions have a value of one. Therefore, when the attenuating window signal is multiplied with the input signal, the peak areas of the input signal is reduced, and all other areas remain the same. However, multiplying the signal in the time domain is equivalent to convolving the spectrum of the input signal with the window spectrum in the frequency domain. This results in a wider spectrum, i.e., spectrum spreading. The window clipping process introduces undesirable spectrum spreading while attempting to reduce the splatter and the P/A ratio of the input signal, and thus, fails to adequately address the problems of the conventional clipping processes.
The present invention is directed to an improved method and device for limiting peaks of an input signal. The method and device isolate peaks of an input signal based on a clipping threshold voltage, generate an extrema signal representing the local extrema of the peak isolated signal, filter the extrema signal to generate a filter signal, and combine the filter signal with the input signal delayed by a predetermined time period to generate an impulse clipped signal. The impulse clipped signal has a reduced P/A ratio and is without significant out-of-band spectrum artifacts.